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    SubjectRe: [PATCH v2 25/39] x86/cet/shstk: Handle thread shadow stack
    On Thu, Sep 29, 2022 at 03:29:22PM -0700, Rick Edgecombe wrote:
    > From: Yu-cheng Yu <yu-cheng.yu@intel.com>
    >
    > When a process is duplicated, but the child shares the address space with
    > the parent, there is potential for the threads sharing a single stack to
    > cause conflicts for each other. In the normal non-cet case this is handled
    > in two ways.
    >
    > With regular CLONE_VM a new stack is provided by userspace such that the
    > parent and child have different stacks.
    >
    > For vfork, the parent is suspended until the child exits. So as long as
    > the child doesn't return from the vfork()/CLONE_VFORK calling function and
    > sticks to a limited set of operations, the parent and child can share the
    > same stack.
    >
    > For shadow stack, these scenarios present similar sharing problems. For the
    > CLONE_VM case, the child and the parent must have separate shadow stacks.
    > Instead of changing clone to take a shadow stack, have the kernel just
    > allocate one and switch to it.
    >
    > Use stack_size passed from clone3() syscall for thread shadow stack size. A
    > compat-mode thread shadow stack size is further reduced to 1/4. This
    > allows more threads to run in a 32-bit address space. The clone() does not
    > pass stack_size, which was added to clone3(). In that case, use
    > RLIMIT_STACK size and cap to 4 GB.
    >
    > For shadow stack enabled vfork(), the parent and child can share the same
    > shadow stack, like they can share a normal stack. Since the parent is
    > suspended until the child terminates, the child will not interfere with
    > the parent while executing as long as it doesn't return from the vfork()
    > and overwrite up the shadow stack. The child can safely overwrite down
    > the shadow stack, as the parent can just overwrite this later. So CET does
    > not add any additional limitations for vfork().
    >
    > Userspace implementing posix vfork() can actually prevent the child from
    > returning from the vfork() calling function, using CET. Glibc does this
    > by adjusting the shadow stack pointer in the child, so that the child
    > receives a #CP if it tries to return from vfork() calling function.
    >
    > Free the shadow stack on thread exit by doing it in mm_release(). Skip
    > this when exiting a vfork() child since the stack is shared in the
    > parent.
    >
    > During this operation, the shadow stack pointer of the new thread needs
    > to be updated to point to the newly allocated shadow stack. Since the
    > ability to do this is confined to the FPU subsystem, change
    > fpu_clone() to take the new shadow stack pointer, and update it
    > internally inside the FPU subsystem. This part was suggested by Thomas
    > Gleixner.
    >
    > Suggested-by: Thomas Gleixner <tglx@linutronix.de>
    > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    > Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    >
    > ---
    >
    > v2:
    > - Have fpu_clone() take new shadow stack pointer and update SSP in
    > xsave buffer for new task. (tglx)
    >
    > v1:
    > - Expand commit log.
    > - Add more comments.
    > - Switch to xsave helpers.
    >
    > Yu-cheng v30:
    > - Update comments about clone()/clone3(). (Borislav Petkov)
    >
    > Yu-cheng v29:
    > - WARN_ON_ONCE() when get_xsave_addr() returns NULL, and update comments.
    > (Dave Hansen)
    >
    > arch/x86/include/asm/cet.h | 7 +++++
    > arch/x86/include/asm/fpu/sched.h | 3 +-
    > arch/x86/include/asm/mmu_context.h | 2 ++
    > arch/x86/kernel/fpu/core.c | 40 ++++++++++++++++++++++++-
    > arch/x86/kernel/process.c | 17 ++++++++++-
    > arch/x86/kernel/shstk.c | 48 +++++++++++++++++++++++++++++-
    > 6 files changed, 113 insertions(+), 4 deletions(-)
    >
    > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
    > index 778d3054ccc7..f332e9b42b6d 100644
    > --- a/arch/x86/kernel/fpu/core.c
    > +++ b/arch/x86/kernel/fpu/core.c
    > @@ -555,8 +555,40 @@ static inline void fpu_inherit_perms(struct fpu *dst_fpu)
    > }
    > }
    >
    > +#ifdef CONFIG_X86_SHADOW_STACK
    > +static int update_fpu_shstk(struct task_struct *dst, unsigned long ssp)
    > +{
    > + struct cet_user_state *xstate;
    > +
    > + /* If ssp update is not needed. */
    > + if (!ssp)
    > + return 0;
    > +
    > + xstate = get_xsave_addr(&dst->thread.fpu.fpstate->regs.xsave,
    > + XFEATURE_CET_USER);
    > +
    > + /*
    > + * If there is a non-zero ssp, then 'dst' must be configured with a shadow
    > + * stack and the fpu state should be up to date since it was just copied
    > + * from the parent in fpu_clone(). So there must be a valid non-init CET
    > + * state location in the buffer.
    > + */
    > + if (WARN_ON_ONCE(!xstate))
    > + return 1;
    > +
    > + xstate->user_ssp = (u64)ssp;
    > +
    > + return 0;
    > +}
    > +#else
    > +static int update_fpu_shstk(struct task_struct *dst, unsigned long shstk_addr)
    > +{

    return 0; ?

    > +}
    > +#endif
    > +

    --
    Sincerely yours,
    Mike.

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